At least 1-1/2 hours before serving, whisk together the olive oil, vinegar, garlic, salt, oregano and a few grinds of black pepper. Rub the mixture into both sides of the steak, cover and refrigerate.
About 30 minutes before you're ready to grill, remove the steak from the refrigerator to bring it to room temperature.
Preheat a gas or charcoal grill to medium-high heat (475 to 500°F) and oil the grates.
Depending on thickness, grill the flank steak for 3 to 4 minutes on the first side, 2 to 3 minutes on the second for medium rare. Transfer the steak to a cutting board and let it rest for 7 to 8 minutes before slicing across the grain.
To serve, transfer the steak to serving platter, drizzle with chimichurri sauce and pass the rest of the sauce at the table.